This invention relates to proofing of semiconductor electronic assemblies, and, more particularly, to a method and apparatus to test and detect failures, both permanent and transient, during environmental test cycling of any solder joints of the electronic module-to-circuit board connections, such as ball grid arrays and/or other electronic module-to-circuit board fastening technology, and indicating such failure.
Given the objectives to be achieved by an electronic assembly, circuit designers may create or select the functional characteristics for the electronic assembly and the hardware to implement the assembly. That design information is turned over to manufacturing engineers who in turn specify or create the manufacturing processes and technology required for mass or volume production of a high quality reliable product that should have a reasonable operational life and production cost. In creating a new electronic assembly, often the circuit design and/or manufacturing technology xe2x80x9cpushesxe2x80x9d the state of the art, so to speak, and one finds there are limits to what is known of a particular manufacturing technology and design.
To advance the knowledge beyond existing limits, the manufacturing engineer necessarily resorts to xe2x80x9cqualificationxe2x80x9d testing of the electronic assembly. That testing particularly involves testing the behavior of a structure over extremes of temperature, and shock and vibration, herein collectively referred to as environmental testing. When the product is intended for critical applications, such as in space vehicles, environmental testing of the technology or technology infrastructure to produce the product is typically accomplished, incrementally, in stages. In so doing, problems in the technological infrastructure revealed by the test may be investigated, isolated and solved, prior to proceeding to a succeeding stage of product development. As each stage of development is so xe2x80x9cproofedxe2x80x9d, one acquires the confidence that the product or electronic assembly when fully developed will perform as intended and is, hence, xe2x80x9cqualifiedxe2x80x9d.
One important stage of such development process is to ensure that the selected soldering technology properly solders the electronic modules to the circuit board. The manufacturing engineer must have or acquire a high degree of confidence that the electronic assembly should not prematurely fail in service due to a defect in the electrical connections, the solder joints, between the electronic module and the circuit board or in the wiring of the circuit board. Typically, for that purpose a prototype or surrogate assembly is fabricated and subjected to the environmental testing. Should the prototype pass those tests, the necessary confidence in the technology is gained. The present invention relates to the foregoing stage of development and testing.
Advanced electronics assemblies used in spacecraft and avionics apparatus incorporate a large number of electronic modules that are very densely packed on a multi-layer circuit board. Because of the dense packing of the electronic modules, the metal interconnections formed on the various layers of the circuit board are necessarily thin, possibly even thinner than 0.001 inch. And the number of layers in the circuit board may be quite large, as many as twenty-four layers not being unusual. The number of modules may exceed one hundred, and the number of module-to-circuit board connections often is greater than four hundred. Some electronic modules may contain 1,000 or more terminals that are connected to corresponding metal bonding pads on the upper surface of the circuit board. For such an application the electronic module structures may be selected from amongst a number of known types, including hermetically sealed types such as ceramic column grid arrays (xe2x80x9cCCGA""sxe2x80x9d), ceramic quad flat pack (xe2x80x9cCQFP""sxe2x80x9d), ceramic dimpled ball grid array (xe2x80x9cCBGA""sxe2x80x9d); or non-hermetic types, such as plastic encapsulated microelectronic circuits (xe2x80x9cPEM""sxe2x80x9d), including plastic ball grid arrays (xe2x80x9cPBGA""sxe2x80x9d), and plastic quad flatpacks (xe2x80x9cPQFP""sxe2x80x9d), all of which are known in the art.
One example of an unknown concerns plastic encapsulated ball grid arrays (xe2x80x9cPBGA""sxe2x80x9d). The PBGA offers a packaging and fastening technology for an electronic module that is less expensive than CCGA""s and CQFP""s. For that reason the PBGA appears most desirable. However, because the PBGA modules have not been used extensively in space based applications, the level of confidence in their reliability in that application is necessarily low as compared to other types of module connections (notably leaded as opposed to leadless types) that have already been proven. Considering that the operation of a spacecraft or a spacecraft electronic system may be jeopardized by even a momentary break in a solder joint in an electronic assembly, the development procedure should ensure that such a break should not be likely to occur. Environmental testing offers the means to raise the confidence level in the foregoing PBGA technology, particularly the confidence of the manufacturer who may be required to offer guarantees to the customer.
The ball grid array portion of an electronic module includes solder balls adhered to the underside of the module. Such modules are typically attached to a circuit board by pre-coating the bonding pads on the circuit board with solder paste, placing the modules loosely into position in the paste, and reflowing the solder paste and solder balls by heating in an infra-red, convection, or vapor phase oven. This electrically and mechanically bonds the solder balls to the associated bonding pads on the circuit board, forming respective solder joints. Because those solder joints are minute in size, are located on the underside and are covered by the package of the module, it is not possible to visually inspect all those solder joints. An indirect means for testing the joints is necessary. The present invention provides the necessary process and apparatus for such testing.
Testing the integrity of solder joints in an electronic assembly or even the wiring of a circuit board is not new. Others have disclosed methods of testing solder joints. U.S. Pat. No. 5,420,500 discloses that a stimulus signal can be capacitively coupled to the component under test to measure the signal level of the solder lead under test. U.S. Pat. No. 5,493,775 discloses that xe2x80x9copensxe2x80x9d in pressure contacts can be detected by transmitting acoustic waves at the solder joint and comparing the harmonics of the received acoustic wave with that of the transmitted acoustic wave. U.S. Pat. No. 6,087,842 and U.S. Pat. No. 6,097,203 disclose that electromagnetic probes may be integrated in an integrated circuit to provide capability for testing continuity between integrated circuits. And U.S. Pat. No. 5,254,953 disclosed that capacitive coupling can be used to detect proper soldering of semiconductor components within a printed circuit board. As becomes apparent, the present invention is believed to be more xe2x80x9cuser-friendlyxe2x80x9d and better suited to testing of large numbers of densely packed inaccessible solder joints and to the detection during environmental testing of breaks in solder joints that are but transient in duration than any of the techniques presented in the foregoing patents.
Accordingly, an object of the present invention is to provide a new and relatively simple apparatus and procedure for indicating transient failure of solder joints and/or electrical wiring in an electronic assembly during environmental testing.
An additional object of the invention is to reduce the time and cost of qualification testing of electronic assemblies.
A further object of the invention is to efficiently proof the integrity of plastic encapsulated ball grid array modules or other such area-array packages during environmental testing. And
A still further object of the invention is to provide a new method and apparatus for detecting and displaying transient discontinuity in electrical wiring of a circuit board.
In accordance with the foregoing objects and advantages, the present invention provides a new procedure and apparatus for qualification testing of electronic assembly fabrication technology in product development for volume production. A test assembly containing a surrogate circuit board and surrogate electronic modules are supported on the circuit board. The terminals of the modules are joined to respective bonding pads on the circuit board to form bonded joints. The terminals of the surrogate modules and the wiring of the bonding pads associated with those modules is configured to produce at least one test series circuit associated with each module, including bonded joints. Such test series circuit may be formed of a row, zone or sub-array of bonding pads. A continuity tester supported on the circuit board monitors the test series circuit and produces a persistent indication to alert the operator of an interruption of that series circuit, should a break or interruption occur in a bonded joint of the series circuit.
To proof the reliability of the bonded joints, the test assembly is subjected to temperature and/or vibration and shock cycling. Should a break or a transient break occur in the test series circuit, the test operator is able to easily determine the existence of a transient break by the persistent indication of the continuity tester.
Further in accordance with the invention, a control unit, located off the circuit board and external to the test assembly, supplies power and operator controls for the test assembly through a test cable. In accordance with an additional aspect to the invention, multiple testers may be included on the circuit board to separately monitor respective series circuits associated with respective modules on the circuit board.
In accordance with a more specific aspect to the invention, the continuity tester comprises a bistable electronic switch and an LED. Normally the bistable switch is placed in one output state. When a break is detected the switch changes to the second output state and supplies energy to the LED, which illuminates.
As an advantage, the continuity tester is mounted in close proximity to the series circuit being tested. Hence, the tester is able to respond to exceedingly brief transients, irrespective of the length of the test cable. As a further advantage, only four off-circuit board connections are required for control of the multiple on-board testers, regardless of the number of testers included in the test assembly. As a still further advantage, because each tester is dedicated to monitor a single series circuit, the tester is able to electronically stare at the series circuit and detect every transient event. The invention provides an effective testing arrangement the implementation of which is very low in cost. With the invention use of expensive off-board scanning testers for long term tests, such as automated thermal cycling lasting many months, is avoided.
The foregoing and additional objects and advantages of the invention, together with the structure characteristic thereof, where were only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.